This invention relates to an idle speed control system for an internal combustion engine, and more particularly, to a system for regulating the idling rotational speed of an engine by adjusting the mass flow rate of fuel delivered to the engine.
Recently, fuel based control strategies have been applied to fuel injected internal combustion engines for air-fuel ratio regulation. In such engines, the amount of fuel injected into each cylinder during an engine cycle is normally determined directly as a function of the operator demand for engine output, as indicated for example, by the degree of depression of an accelerator pedal. In response to the quantity of fuel injected into each cylinder, the engine intake air flow is then controlled in a closed-loop fashion to achieve the appropriate engine air-fuel ratio.
Idle speed regulation in engines operating according to a fuel based control strategies has conventionally been performed by directly adjusting the quantity of fuel injected into each cylinder during the engine cycle, since known idle control techniques based on air flow adjustment are not applicable. This is typically accomplished by employing well known proportional-integral-derivative (PID) control, or some variation thereof, to adjust the quantity of fuel injected per cylinder per cycle in accordance with the difference between the actual engine idling speed and a desired engine idling speed to reduce the difference between the desired and actual idling speeds.
In practice, the above-described approach for fuel based idle control has exhibited instability under certain engine operating conditions. It has been found that this instability results because of the nature of relationship between engine speed and the engine parameter being adjusted, i.e. the quantity of fuel injected per cylinder per cycle. This particular engine parameter does not behave in a monotonic fashion with regard to engine rotational speed. At low engine speeds, the quantity of fuel that must be injected into each cylinder to sustain a constant rotational speed initially decreases with increasing idling speed. This is due to the improved thermal efficiency and scavenging of the engine as rotational speed increases. Eventually frictional losses in the engine rise to the point where the quantity of injected fuel per cylinder then has to be increased to achieve an increase in engine speed. Due to this non-monotonic behavior, traditional idle control systems are not able to quickly and accurately adjust the quantity of injected fuel to correct for idling speed errors. Depending upon the rotational speed of the engine, such a correction to the quantity of injected fuel can be too small, too large, or even in the wrong direction. As a result, systems using the conventional approach for fuel based idle speed control are prone to speed hunting and complete instability at certain engine idling speeds.